Ceramics have excellent physical properties such as a low heat expansion coefficient, high heat conductivity, electric insulation, and the like. A printed wiring board and other various products are produced and practically used by applying these excellent physical properties.
Since, ceramics generally have poor machinability, it is necessary to use a special machine or device and a high machining technique in order to manufacture a desired part from ceramics. The so-manufactured parts are therefore expensive, and the ceramics are therefore extremely limited in use.
A machinable ceramic has been therefore developed. The machinable ceramic is improved in machinability to a great extent depending upon the kinds of inorganic powders used, a combination thereof and an amount ratio thereof, while the improvement in machinability has its own limits. For example, it is generally difficult to cut the machinable ceramic to a relatively wide board having a thickness of about 1 mm or to drill holes at narrow pitches. Even if the above machining is possible, the produced part is fragile so that it is exceedingly limited in use, if it is used as is.
Further, the above machinable ceramic generally has pores. As a consequence, the defect thereof is that it suffers a great change in physical properties due to humidity absorption when used in usual environments.
On the other hand, attempts have been made in various ways to form copper-clad boards by using, as a substrate, of porous ceramic sintered substrates (U.S. Pat. No. 4,882,455, JP-A-64-82689 (=JP-B-3-50429)).
However, the above prior art references essentially require using a glass cloth in combination, or some other means, and describe nothing concerning the technique relating to a metal-foil-clad composite ceramic board obtained by impregnating a porous ceramic sintered substrate with a resin, placing a metal foil directly on the resin-impregnated substrate and laminate-molding the resultant set.
The present inventors already filed patent applications (U.S. Pat. No. 5,531,945, JP-A-6-152086, etc.) of inventions directed to a method in which an inorganic continuously porous sintered body (porous ceramic sintered substrate) is impregnated with a thermosetting resin to form a resin-composite ceramic and the resin-composite ceramic is sliced.
Further, in the course of subsequent studies of the above method, there were found a novel metal-foil-clad composite ceramic board and a process for the production thereof, in which a metal foil is strongly bonded to a composite ceramic board through a minimum amount of an adhesive layer by concurrently carrying out the bonding of the metal foil and the fabrication of the resin-composite ceramic layer, and a patent application therefor was filed (Japanese Patent Application No. 7-310568).
Meanwhile, the above composite ceramic board or metal-foil-clad composite ceramic board that is easily machinable with a fabricating machine used for a glass epoxy laminate is limited with regard to hardness and strength, as shown in the specification of the above patent application. For example, a sintered body of cordierite is studied in various ways since it can be produced at a relatively low cost and has a low heat expansion coefficient. Generally, however, the sintered body of cordietie is difficult to machine with a fabricating machine used for a glass epoxy laminate. Quartz is a material which has a dielectric constant of approximately 3.5 to 4 and a dielectric loss tangent of 10.sup.-4 or less and is feasible for use in the range of from a wave range of centimeter wave to millimeter wave. However, a sintered body of conventional porous quartz is hard and fragile, and it is difficult to machine even if it is hardened by impregnating it with a resin.
The present inventors have made diligent studies to overcome the above problems and have accordingly arrived at the present invention.